As is known, organic optoelectronic devices such as OLEDs, devices comprising organic photovoltaic cells (OPVCs) and those comprising organic transistors (TFTs) need to be encapsulated in order to protect their sensitive components from gaseous species in the atmosphere (mainly oxygen and water vapor). Specifically, if this protection is not provided there is a risk that the device will subsequently degrade, which degradation manifests itself mainly via the appearance of dark black spots in the case of OLEDs, which spots are in fact the result of the penetration of water vapor into the diode, which penetration degrades the interface between the cathode (or anode) and the organic film or films.
This encapsulation may typically be achieved using a glass cap that is adhesively bonded to the organic device using a specific adhesive, especially one having a low water permeability. A solid moisture absorber or getter is generally added between the substrate and the cap in order to prolong the lifetime of the device.
For certain applications, but also in order to reduce cost, thin inorganic barrier films have been developed, the role of which, analogously to that of the cap/getter assembly, is to protect the underlying device from attack by moisture. Generally, these barrier layers are oxides (such as preferably Al2O3) nitrides or oxynitrides, or in certain cases they may be thin metal films, except if the light-emitting unit emits via the top of its structure (i.e. top emission) in which case the barrier layers must be transparent.
These thin films are deposited by standard vacuum deposition processes such as chemical vapor deposition (CVD), which is optionally plasma enhanced (PECVD), atomic layer deposition (ALD, sometimes called ALCVD), or by physical vapor deposition (PVD) processes including evaporation and sputtering. For the barrier layer, CVD and, in particular, ALD technologies are preferred, which technologies, at low temperatures, give dense barrier layers with few defects (pinholes), these layers being 100% conformal at temperatures most frequently below 110° C. i.e. at temperatures compatible with OLEDs. Thus, at low temperatures a defect density as low as 38/cm2 has been demonstrated in an Al2O3 layer deposited by ALD. For microdisplay applications, this defect density is nevertheless too high because, for a microdisplay area of 45 mm2 this would lead to 17 defects per microdisplay, i.e. to potentially 17 dark spots in the OLED display. Specifically, even though these dark spots intrinsic to the fabrication process are microscopic, their presence is prohibitive in an OLED device the image of which is magnified by suitable optics, and it is furthermore necessary to add the “extrinsic” dark spots due to the presence of undesirable particles present on the surface of the device when it is encapsulated in these thin films.
Moreover, it is known that it is necessary to protect ALD-deposited Al2O3 from water over the long term because it has a tendency to hydrolyze into Al(OH)x. It has therefore been sought to durably passivate the barrier formed by such an Al2O3 film using more chemically inert and stabler inorganic material such as SiO2, Si3N4 or SiOxNy, via deposits produced by low-temperature PECVD, which deposits also allow the residual defects in these Al2O3 films to be filled.
As a variant, it has been sought to passivate these ALD-deposited Al2O3 films with a number of relatively thick planarizing polymer-based organic layers, which layers are intended to remedy the drawback of the aforementioned undesirable particles by coating them with a multilayer of alternating organic and inorganic layers, in the manner of the Vitex Barix™ multilayer. One drawback of this solution lies in the “flash” gas-phase evaporation process used (i.e. the monomer is evaporated, condensed on the substrate, and then exposed to UV in order to cross link it) which process is relatively costly in time.
It is difficult to envision using other less time-costly types of deposition, such as liquid phase deposition, because this type of deposition requires the use of polymer solutions, these solutions containing solvents that are liable to dissolve the layers of the underlying light-emitting unit.
The patent application FR 10 01522 filed 12 Apr. 2010 in the name of the Applicant discloses an organic optoelectronic device, for example an OLED, that remedies these drawbacks, this device comprising a light-emitting unit comprising an active zone coated with an airtight multilayer encapsulation structure and an adjacent electrical connection zone, the encapsulation structure comprising at least one inorganic film multilayer F1/photoresist layer C1 where the film F1 covers the active zone and is surmounted by the layer C1, which layer C1 is deposited in liquid phase, etched by photolithography and covers the film F1 while extending around the active zone in a structured enveloping portion that terminates short of the connection zone, hence the layer C1 passivates the film F1 and provides the active zone with lateral protection from the solvents and developer solutions used to etch the layer C1. The latter is advantageously covered with an external inorganic barrier film Fe that surmounts both the active zone, while completely covering the layer C1, and its connection zone, so as to isolate from the exterior the enveloping portion of the layer C1.
This patent application also mentions the possibility of carrying out, after the enveloping resist layer has been developed, dry etching of this layer, for example reactive ion etching (RIE) or etching in an oxygen plasma, in order to decrease its thickness and therefore the total thickness of the multilayer, and thus meet the specifications of microdisplays in respect of crosstalk (i.e. in order to prevent emission from neighboring pixels overlapping when color filters are used).
The Applicant has observed that in certain cases, for example for large display sizes, or following certain heat treatments, blisters may appear on the surface of the external film covering the resist (see FIG. 1, appended to the present description). These blisters seem to increase in number as the volume of the underlying resist increases. The appearance of blisters therefore seems to be related to a resist degassing effect. To mitigate this drawback the resist may be thinned (in order to decrease the total volume thereof) for example by using a plasma.
However, one drawback of this solution consisting in thinning the photoresist to a significant degree in order to prevent this blistering, lies in the risk that results therefrom that thickness uniformity will be penalized between the edges and the center of the device and at the integrated-circuit scale (because of nonuniformity between the edges and center). This risk is all the more marked the larger the size of the device. In addition, the barrier properties of the encapsulation may be penalized by such thinning of the resist.